## Questions and Answers

What is the main advantage of using a shunt resistor in a moving coil ammeter?

It increases the current capacity of the instrument

Why is a permanent-magnet moving coil (PMMC) instrument impractical for measuring large currents directly?

The current capacity of the PMMC is too small

What is the relationship between the shunt resistance ($R_{sh}$) and the internal resistance of the movement ($R_m$) in a moving coil ammeter?

$R_{sh} = R_m / (I / I_m - 1)$

Which of the following is NOT a desirable property of a shunt resistor for a moving coil ammeter?

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What is the purpose of the 'multiplying power of the shunt' ($I/I_m$) in a moving coil ammeter?

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How can the current range of a moving coil ammeter be extended to measure higher currents?

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What is the main advantage of a moving coil instrument compared to other types of instruments?

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Which of the following equations correctly represents the relationship between the full-scale voltage ($V$), the deflection current ($I_m$), the internal resistance ($R_m$), and the series resistance ($R_s$) for a voltmeter?

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What is the purpose of using a shunt resistor in an ammeter?

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Which of the following equations correctly represents the relationship between the shunt resistance ($R_{sh}$), the internal resistance ($R_m$), and the full-scale deflection current ($I_m$) for an ammeter?

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What is the main disadvantage of using a moving coil instrument for high-voltage measurements?

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Which of the following equations correctly represents the relationship between the full-scale voltages ($V_1$, $V_2$, $V_3$, $V_4$) and the internal resistance ($R_m$) for a multi-range voltmeter?

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What is the main purpose of the two springs being spiraled in opposite directions in a moving coil instrument?

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Which of the following is NOT an advantage of a permanent-magnet type moving coil instrument?

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Which of the following is a disadvantage of permanent-magnet type moving coil instruments compared to moving-iron instruments?

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What is the main purpose of using a shunt with an ammeter?

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Which of the following is NOT a desirable property of a good shunt for an ammeter?

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## Study Notes

### Ideal Shunt Characteristics

- The resistance of a shunt should not vary with time.
- Shunts should carry current without excessive temperature rise.
- They should have a thermal electromotive force (EMF) with copper.

### Ammeter Shunt

- A PMMC (Permanent Magnet Moving Coil) is used as the indicating device in an ammeter.
- The current capacity of a PMMC is small, making it impractical to construct a PMMC coil that can carry a current greater than 100 mA.
- A shunt is required for measuring large currents in an ammeter.

### Shunt Calculation

- The voltage drop across the shunt (Rsh) and movement (Rm) must be the same.
- The equation to calculate the shunt resistance (Rsh) is: Rsh = Im * Rm / (I - Im)

### Multi-Range Ammeter

- A multi-range ammeter uses multiple shunts with different multiplying powers (m) to measure different ranges of currents.
- The shunt resistance for each range is calculated using the equation: Rsh = Rm / (m - 1)

### Voltmeter

- A series resistor or multiplier is required to extend the range of a voltmeter.
- The equation to calculate the multiplier resistance (Rs) is: Rs = (V - Im * Rm) / Im

### Multi-Range Voltmeter

- A multi-range voltmeter uses multiple multipliers with different multiplying powers (m) to measure different ranges of voltages.
- The multiplier resistance for each range is calculated using the equation: Rs = Rm * (V - Im * Rm) / Im

### Examples

- Example 1: Calculating the shunt resistance for an ammeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
- Example 2: Calculating the series resistance for a voltmeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
- Example 3: Calculating the full-scale reading of a PMMC instrument.

### PMMC Instrument Advantages

- They have low power consumption.
- Their scales are uniform and designed to extend over an arc of 170°.
- They possess a high (torque/weight) ratio.
- They can be modified with the help of shunts and resistances to cover a wide range of currents and voltages.
- They have no hysteresis loss.
- They have very effective and efficient eddy-current damping.
- They are not much affected by stray magnetic fields.

### PMMC Instrument Disadvantages

- They are somewhat costlier due to delicate construction and accurate machining and assembly of various parts.
- Errors set in due to the aging of control springs and the permanent magnets.
- They are mainly used for DC work only, but can be used for AC measurements with rectifiers or thermo-junctions.

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## Description

Learn about the important characteristics and functions of shunt resistors used in ammeters. Understand why shunt resistors should not vary with time, carry current without excessive temperature rise, and have a thermal electromotive force with copper. Explore the necessity of using shunt resistors to measure large currents in ammeter circuits.